Field of the Invention
The presently disclosed invention relates to electrical switches and, more particularly, electrical power transfer switches.
Discussion of the Prior Art
Electrical power transfer switches have been used to transfer an electrical load from one power source to another power source. Frequently, such switches are used in emergency panels that transfer incoming line power to an emergency generator or other source at times when the standard power source has been interrupted or failed due to inclement weather or other emergency conditions such as flooding.
In the prior art, transfer switches have been developed to reliably and automatically switch industrial and commercial loads such as factories, shopping malls and hospitals to an alternate power source in the event of an electrical power failure. Many examples are known in the prior art.
Such transfer switches have worked well, but their cost and size did not lend their application to light commercial or residential use. Accordingly, there was a need in the prior art for electrical power transfer switches that would meet all UL and other applicable standards for reliability and safety, but that were less costly and more adaptable for use in lighter duty applications such as in small businesses and homes.
Some power transfer switches that have been used in the past have been relatively difficult to assemble. Further, their design is not readily adaptable to modification or multiple application. Examples are shown in U.S. Pat. Nos. 6,538,223 and 8,735,754. U.S. Pat. No. 6,538,223 describes a transfer switch wherein contacts to a load can be toggled between oppositely opposed supply contacts that are connected to respective power supplies to switch from one power supply to another. The load contacts are located on opposite faces of an arm that is moveable between the two power contacts to electrically connect the load contacts with one of the power contacts. The arm is connected to a cross bar that is reversibly rotatable through an arc in clockwise and counterclockwise directions to move the arm into one position where the load contacts engage the contacts of the first power source and a second position in which the load contacts engage the contacts of the second power source. The cross bar includes two extending members that are connected to respective plungers of two solenoids such that the angular position of the cross bar is controlled by extension and retraction of the solenoid plungers.
Transfer switches are subject to a well-known phenomenon known as “blow open” wherein opposing electrical fields of the load contacts and the supply contacts tend to be forced apart as the contacts are brought into proximity. To overcome this difficulty, the cross bar in U.S. Pat. No. 6,538,223 is caused to over-rotate the end points of the arc that is necessary to bring the load contacts and the power contacts together and the load contacts are spring loaded to mechanically absorb the interference between the load contacts and the supply contacts. In the structure of U.S. Pat. No. 6,538,223, a spring biases the arm against a stop. That design causes the arm to develop separate fulcrum points (and therefore different closing force) between the load contacts and the supply contacts depending on the angular direction of the cross bar.
U.S. Pat. No. 8,735,754 shows an alternative mechanism for the spring bias of the load contacts against the supply contacts. In that patent, the spring bias force for the load contacts is directed along the plane of the arm so that the arm rocks across the center axis of the spring by the degree of over-rotation.
It has been found that prior art designs such as shown in U.S. Pat. Nos. 6,538,223 and 8,735,754 were limited to specific applications according to their particular design. Also, it has been found that the assembly of transfer switches according to those designs was somewhat difficult and costly. For example, in the designs of U.S. Pat. Nos. 6,538,223 and 8,735,754 the springs that spring bias the load contacts against the supply contacts have a relatively high spring force so that compressing the springs to form a finished assembly was difficult and required special tools or jigs.
Accordingly, there was a need in the prior art for a transfer switch that could be assembled easily and without special tools and that also was adaptable to various applications.